In the past, many attempts have been made for controlling the output power, either of the current or voltage of a storage battery. The direct current is usually controlled by including a variable resistor in the output circuit. This, of course, causes power losses, the power being converted into heat. Also, a direct current may be converted into an alternating current. If a square wave output is desired, the efficiency of the conversion can be rather high. On the other hand, if an alternating current must be in the form of a sine wave, the loss approximates 30 percent, since it is usually required to produce a square wave first, which may then be converted into a sine wave by a suitable network.